The pulse-height analyzer here contemplated comprises circuitry to which the output pulses of the detector are fed, generally after suitable amplification, and which selectively addresses the cells of a multicell memory according to the digitized pulse amplitudes. In prior spectroscopic systems the contents of any cell so addressed are incremented by a value of unity; at the end of the observation period, therefore, the several memory cells contain respective counts of incident particles pertaining to different energy levels or transit times which may be collectively referred to as "channels". These counts, however, are incomplete since pulses arriving during the so-called "dead time" of the analyzer (while a preceding pulse is still being processed) are not registered; in the absence of special precautions, moreover, the counts are affected by pile-up of closely spaced detector pulses whose overlap may cause them to merge into a single pulse with a false peak amplitude.
It has already been proposed to compensate for these counting losses by replacing the aforementioned increments of unity value with different numerical values, termed weighting factors, taking the dead time of the analyzer into account; see article by J. Harms in Vol. 53 of Nuclear Instruments and Methods, page 192, published 1967 by North-Holland Publishing Company of Amsterdam, Netherlands. In order to minimize the effect of pulse overlap, pile-up rejectors have been developed which prevent the analyzer from responding to more than one detector pulse at a time; such a pile-up rejector has been described, for example, in my article titled A HIGH RATE GAMMA SPECTROSCOPY SYSTEM FOR ACTIVATION ANALYSIS OF SHORT-LIVED ISOMERIC TRANSITIONS which appeared in Vol. 136 (1976) of Nuclear Instruments and Methods, pages 271-283. Aside from a rather complex circuitry, however, these devices depend for efficient operation on a certain pulse shape and are also susceptible to false triggering by electrical interferences.
A technique designed to correct for both dead time and pile-up uses a series of test pulses which are introduced in parallel with the detector pulses into the preamplifier of the system. The ratio of the number of test pulses registered correctly by the analyzer to the total number thereof generated during the observation period is taken as representative of the proportion of properly processed detector pulses; thus, the reciprocal value of this ratio serves as a corrective factor by which the number of events written in each memory cell at the end of the observation period must be multiplied in order to yield the true channel counts. Reference in this connection may be made to articles by M. O. Deighton and by E. J. Cohen which appeared in Vol. 14 (1961), page 48, and Vol. 25 (1974), page 25 , of Nuclear Instruments and Methods.
A problem of this known procedure resides in the fact that, on the one hand, the test pulses are themselves a cause of counting losses and ought therefore to be widely spaced whereas, on the other hand, a high pulse cadence is desirable for a more exact determination of the corrective factor. These test pulses, moreover, originate at a common source and therefore cannot overlap so that their loss rate is not the same as that of the detector pulses which are subject to pile-up; this drawback is particularly noticeable with high pulse cadences. Furthermore, the amplitude of the test pulses should fall within a gap of the pulse-height spectrum which must be determined by a preliminary scan in advance of the actual measurement; such determination can be made only by a skilled operator and must be individually carried out in each instance, thus precluding the utilization of this technique for series examinations or with automatic equipment. Finally, an evaluation of counting losses by this method can be performed only at the end of the observation period during which the cadence of the test pulses must be held constant; the method is therefore inapplicable to the measurement of short-lived nuclear particles or of radiation sources pulsed with time-varying frequencies.